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Unformatted text preview: ECE ECE 3534 Digital Design 1
Section 5a: Review of FlipFlops Flip FlipFlops
• We can describe flipflops in terms We of their characteristic information. • Characteristic information describes the next state of the flipflop as a function of the present state and all of the flipflop inputs. • Instead of considering the operation of the flipflop in terms of all of the distinct combinations ll di of input and present state, we can describe the flipflop in terms of the functions that each combination of the inputs carries out. The The SR FlipFlop
S 0 0 1 1 R 0 1 0 1 Q Q+ 0 1 0 1 0 1 0 1 Function The characteristic equation for the SR flip flipflop is Q+ = S + R′Q, but this but characteristic equation only applies for valid values of S and R. The The D FlipFlop
Imagine Imagine that an SR flip flop has had its S and R inputs connected through an inverter. Call the new input D.
S 0 1 R 1 0 D 0 1 Q Q+ 0 1 0 1 Function The characteristic equation of the D flipflop is Q+ = D. The The JK FlipFlop
Consider Consider the JK flipflop as an improved SR flipflop. Unlike the SR flipflop, the input J = K = 1 has a valid function. lid
J 0 0 1 1 K 0 1 0 1 Q Q+ 0 1 0 1 0 1 0 1 Function The characteristic equation for the JK JK flipflop is Q+ = JQ′ + K′Q. The The T FlipFlop
Imagine Imagine that a JK flip flop has had its inputs connected. Call the new input T.
J 0 1 K 0 1 T 0 1 Q Q+ 0 1 0 1 Function The characteristic equation for the T flipflop is Q+ = T ⊕ Q. Flip FlipFlops
• Characteristic information answers Characteristic the question “Based on the present information, what will the next state be?” • This information is important, but it doesn’t answer an important design question. Generally, we know what the present state is and what the next state should be. • Our question is “Given a known present state, and a desired next state, what do the flipflop inputs have to be to achieve the state transition? transition?” Excitation Excitation Information
• This information is called excitation This information. • We can derive the excitation information for each flipflop by reorganizing its characteristic information. • If we consider the transitions as functions, we can determine which functions of the flipflop achieve fli each transition. Excitation Excitation Information
SR flipflop • Functions: no change, set, reset Function? Q 0 0 1 1 Q+ 0 1 0 1 S R Excitation Excitation Information
D flipflop • Functions: set, reset Function? Q 0 0 1 1 Q+ 0 1 0 1 D Excitation Excitation Information
JK flipflop • Functions: no change, set, reset, toggle
Function? Q 0 0 1 1 Q+ 0 1 0 1 J K Excitation Excitation Information
T flipflop • Functions: no change, toggle Function? Q 0 0 1 1 Q+ 0 1 0 1 T ...
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 '06
 JSThweatt

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